A reduction in the toughness, in particular, impact absorption energy of steel products as the number of non-metal inclusions in steel is increased has often been investigated. For example, Non-Patent Document 1 describes MnS. In the case of carbides, Non-Patent Document 2 describes the effect of primary carbides in tool steel. The relationship between the non-metal inclusions and the impact absorption energy is generalized by regarding the non-metal inclusions as vacancy-type defects and is studied as the relationship between the defect size in steel and the impact properties. It appears that the impact properties are reduced as the size of the inclusions is increased.
Meanwhile, with respect to electric resistance welded seams, oxides referred to as “penetrators”, specifically, minute inclusions (in the form of an ellipse with a longitudinal diameter of 0.2 to 0.5 mm) present in welding faces by electric resistance welding, have been reported (Non-Patent Document 3). In general, impact properties of electric resistance welded seams have been said to be poor because of the presence of such penetrators. For the purpose of improving impact properties of electric resistance welded seams, there have been advances in a technique for reducing such penetrators. For example, heat input control relying on experience has been performed.
Disadvantageously, just reducing penetrators, which has been reported, does not necessarily improve impact properties.    Non-Patent Document 1: “Tekkou to Goukin Genso (ge) (Steel and Alloy Element II)”, 1st ed.; edited by the 19th Committee for Steelmaking, Japan Society for Promotion of Science; published by Seibundo Shinkosha Inc.; 25 Mar. 1966, pp. 165-274 (in particular, pp 191-208).    Non-Patent Document 2: “Hagane no Kyoujinsei (Toughness of Steel)”; edited by Iron and Steel Institute of Japan and Japan Institute of Metals; published by Climax Molybdenum Development Company (Japan) Ltd.; 1971, p 207.    Non-Patent Document 3: “Chouonpa Tanshou Series II, Yousetsu Koukan no Chouonpa Tanshou (Ultrasonic Testing Series II, Ultrasonic Testing for Flaw Detection of Welded Steel Pipe)”; edited by Iron and Steel Institute of Japan; published by Iron and Steel Institute of Japan; 1988, p 28-31.